1. Field of the Invention
This invention relates to a light source of the image reading apparatus.
2. Description of the Related Art
The multifunctional printer combining functions of the copying machine, the scanner, the printer, and the facsimile machine, is provided with an image reading apparatus that optically reads images illustrated on an original.
The reduction optical system (the reduction CCD System) has been well known as the above-mentioned image reading apparatus. The reduction optical system has a merit that it is possible to obtain clear images by making the focal depth of the lens large even in case where the original is lifted from the original plate, and meanwhile, there is a demerit that such mechanism made the apparatus large. Therefore, in case of considering the downsizing and thin body of the apparatus, the contact type optical system is adopted in general wherein the information of the original is led to a sensor 108 with the same magnification of the erected image, as shown in FIG. 22.
Specifically, two LED arrays 112 are placed symmetrically each other over the original surface 106 at a specific oblique angle. At the position over a center between two LED arrays 112, a rod lens array 121 is placed. The rod lens array 121 receives light emitted on the original surface 106.
The LED array 112 is formed by arranging plural LED elements 125 on a substrate 124 as shown in FIG. 23. The rod lens array 121 is formed by arranging a specific number of a cylindrical rod lens 122 adjacently and holding them by the substrate 124 as shown in FIG. 24.
Using such contract type optical system configured as above makes the distance between the original surface 106 and the rod lens 122 short, therefore it is possible to downsize the apparatus appreciably.
In order to downsize the apparatus, this is also an important thing that the light source is placed at a position as closer to the original surface as possible. However, since the conventional type of the LED array is an aggregate of point sources, this light source must have a specific distance from the original surface in order to ensure the luminance uniformity. That is to say, there is a limit to promote the downsizing of the apparatus by using the conventional LED array.
Therefore, the applicants of this invention have disclosed in Japanese Patent Application No. 2000-217561 that the following electro luminescence film is used as the area light emitting source.
Specifically, as shown in FIG. 25, a transparent electrode film 103 is formed in the main scanning direction on the transparent substrate 101 of a long glass or transparent resin, on which the electro luminescence film 100 as the light medium is formed, and finally a metal electrode 102 is laminated on the upper surface.
In addition, in case where such area light emitting source is carried out in color, since the illuminance must be uniformized in the main scanning direction, the electro luminescence films 100r, 100g and 100b in the same width corresponding to each color of R (red), G (green), and B (blue) are formed in the sub scanning direction, as shown in FIG. 26.
As shown in FIG. 27, two area light emitting source 5 are arranged symmetrically over the original 9 with keeping a specific interval. Thereby the light emitted on the original is led to the sensor 1 through the lens 14 placed over the center between two area light emitting sources 5.
Even if the area light emitting source comes up to a reading position Pa, the uniform illuminance can be obtained at this reading position Pa. Therefore, it is possible to promote the downsizing of the apparatus by using the area light emitting source 5 proposed by the applicants instead of the conventional LED array.
However, there is a problem that the area light emitting source adopting the electro luminescence film 100r, 100g and 100b has a short life. As shown in FIG. 26, if somewhere, a point X, has a defect like the film thickness is thin, the current gathering at this point X of which the resistance value is low, and the electro luminescence film 100b burns out from this point.
The invention is proposed based on the above-mentioned conventional problems, and has an object to extend the life span of the light source of the image reading apparatus while uniformizing the illuminance in the main scanning direction.